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03.05.11: Notes on “Rapid changes in mixed layer stratification driven by submesoscale instabilities and winds” by Mahadevan et al. (2010)

“Rapid changes in mixed layer stratification driven by submesoscale instabilities and winds”

“The oceanic mixed layer (ML) is maintained through a competition between processes that mix away the stratification and those that restore it. Fluxes of momentum, heat, and salt, induced by winds, cooling, and evaporation at the air‐sea interface, drive shear and convective instabilities that lead to turbulent mixing and reduce stratification. Conversely, precipitation and heating at the surface cause restratification. In this one‐dimensional scenario, the ML deepens when surface waters become denser and trigger vertical mixing or when surface stresses drive shear instabilities. Conversely, it shallows when heating and precipitation reduce the surface density.”

About SST gradient in the ocean: see Weller et al. (2004; DSRI, 665-699)

“Similarly, when winds blow over lateral density fronts, they can lead to restratification by advecting light water over dense or, conversely, destratification by advecting dense water over light.”

“Rossby and Richardson numbers are close to one, and hence, they differ dynamically from interior mesoscale eddies [Thomas et al., 2008].”

See observations of Hosegood et al. (2006; GRL): “Sub‐mesoscale lateral density structure in the oceanic surface mixed layer”

“the largest concentration of stratification is acquired at middepth in the ML.”

“The stratification generated is a factor of 10 larger than expected from geostrophic adjustment of the front.” The fact that the re-stratification events in the HYCOM and OFES simulation are associated with submesoscale vertical component of velocity suggests that the instabilities are not geostrophic adjustment as those described by Tandon and Garrett (1994).

According to the linear theory developed by Fox-Kemper et al. (2008), the deeper the SML, the faster and stronger the re-stratification.

“Unlike heating, which changes the mean buoyancy, MLEs change N2 by redistributing buoyancy.”

Ideas:

  1. On top of looking at the strength of the wind, look at the angle between the wind and the SST front.
  2. Use their ratio to determine which mechanism between wind-induced stratification and density-gradient induced re-stratification dominates. The ratio involves the wind stress, the mixed layer depth, the background density and the horizontal buoyancy gradient.